This has to be still standardized in case of duckweed which will further increase the yield. Moreover, most of the literature survey was obtained from areas of the world (e.g. North Carolina, USA) where the climatic conditions for duckweed cultivation all through the year might be less favourable than those in India. The first part of our article we finished with the following remarks: “Thus, we can produce very effectively huge amount of biomass from aquaculture of duckweed and during duckweed cultivation the waste water can be cleaned. The next important question arises: What can be done with this duckweed biomass? What are the harvested duckweeds good for?” This is what we want to present in this second part of our article.

Do you remember why these plants in English are called “duckweeds”? The most evident reason is that ducks love to eat them; but in reality also other water birds like geese and swans feed on duckweed when available. Consequently, farmers tried to feed these plants to breeding animals and were successful with chicken, cows, pigs, sheep, goats, rabbits, nutria, muskrats, and some types of fishes like Tilapia and carps. Feeding chicken with duckweed biomass might be an attractive possibility in India, with special reference to an extensive investigation by researchers from the North Carolina State University in Raleigh, USA. A certain part of the (high level; 12.5 %) daily food was replaced by duckweed cultivated on effluent from anaerobic digestion of swine wastewater. No negative effects were observed on chickens or on the eggs produced from these chickens. In contrast, the concentration of the Omega 3 fatty acids was slightly enhanced and the carotenoid content increased strongly in these eggs giving a deep yellow- orange colour to the yolk which is attractive to the customers. In cases where the chickens are undernourished, a simple inclusion of duckweed in their diet might increase the egg laying capacity. Similarly, positive effects were found on quantity and quality of milk when duckweed was fed to milk cows.

Duckweed is also used for human nutrition as it is well-known from Laos, Cambodia and Thailand as poor people’s food. However, there is no reason to restrict it to poor people as investigations showed it meanwhile. Duckweed can be considered as healthy food, rich in vitamins and has more than 40 % protein on a dry weight basis. Moreover, the composition of the protein is also valuable for human nutrition. As an example, the content of Lysine is closer to the human diet demand than most other plants. Nobody would get the idea to live exclusively on duckweed but it is highly recommended as a supplement to daily food. An important precondition, however, is that the plants are cultivated under acceptable hygienic conditions. Wastewater, highly contaminated with bacteria (e.g. Escherichia coli), fungi or toxic compounds cannot be used for this application. It is obvious that this increases the input costs, nevertheless, might have an attractive market keeping in mind the advantages. The high protein content is the basis for use of duckweed as human food and animal feed supplement.

Because of the strong pressure in modern day’s economy to exclusively use renewable energy (see the first part of our article), many plants like maize, sugarcane and rape are used to produce energy instead of food resulting in a shortage of arable land available for the agricultural food production. As a consequence, prices for food increased dramatically in many regions of the world. Also here duckweeds represent an attractive alternative to terrestrial plants as their production does not depend on the availability of arable land. As mentioned above, duckweeds when grown under favourable conditions are rich in proteins. Contrastingly, when duckweeds are under certain stress they tend to accumulate starch although the protein content goes much lower. Following this, a two step procedure has been developed: initial cultivation of duckweeds under optimal growth conditions producing large amount of biomass and then, as a second step subjecting them to certain stress which results in inhibition of growth but not photosynthesis of the green plants (as proven from our own research). As a consequence, the sugars produced by plants during photosynthesis are not anymore used for growth but are accumulated as starch.

So, how can we create stress conditions that stop plant growth but not photosynthesis in duckweeds? We looked at the effects of nutrient (phosphate, nitrate, sulphate) limitation, salinity and heavy metal stresses on growth of duckweeds. In all three cases the growth was blocked but not photosynthesis, accumulating a maximum of 50% starch on dry weight basis. This is less than that in potato tubers (at least 70 %) but in the case of duckweeds the whole green plants accumulate starch in contrast to only some parts of the plant e.g., tubers in potato. Why is starch accumulation in plants interesting to us? The starch present in the harvested plants can be degraded back to sugars which can be fermented to alcohol. This alcohol (bio-ethanol) can be used as bio-fuel for cars or in other combustion engines to supply energy. This conversion process is under further research for its commercial viability for instance, at Rutgers University in New Brunswick, USA and at Chengdu Institute of Biology, Chinese Academy of Sciences, China. This might be a promising development especially when the production of duckweed biomass is combined with waste water purification.

One other possible use of duckweed biomass is production of biogas. Biogas can be produced from any organic material although with different efficiencies. Organic material is transformed by microorganisms into methane and carbon dioxide in the absence of oxygen. Methane thus produced can be used as bio gas to produce energy. Duckweed is especially well suitable for this purpose as it contains very small amounts of lignin (a major component of wood) which otherwise would make microbial degradation difficult. The resulting liquid waste after bio gas production still contains almost all nutrients from the green plants and can be further used, after dilution with fresh water, as fertilizer for duckweed growth.

We are sure that in India more opportunities might exist for use of duckweed than we could describe here. We hope that our articles give an insight into the potential of duckweeds and initiate the thought process of readers towards the use of this still mainly unused crop plant.

Views expressed by PD Dr Klaus-J Appenroth, University of Jena, Germany ( and Dr K Sowjanya Sree, AIMT, Amity University, Noida, UP, India (